Make extended `get` results a named tuple

You can now access the returned tuple of a get operation
with names:

    result = cb.get('key', extended=True)
    # result.value, result.flags, result.cas

Change-Id: If156aa60678295749fb7cf3ef3108edc0800f789
Reviewed-on: http://review.couchbase.org/25734
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Reviewed-by: Cosmin Luță <q4break@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>